Electrical heating appliance of the open-grid or boiling-plate type



Dec. 18, 1934. N. w. GILBERT ELECTRICAL HEATING APPLIANCE OF THE OPEN GRID 0R BOILING PLATE TYPE Filed March 16, 1933 2 SheetsSheet 1 wmlyw {NVZNTO'R N. W. GlLBERT Dec 18, 1 934.

ELECTRICAL HEATING APPLIANCE OF THE OPEN GRID OR BOILING PLATE TYPE 2 SheetsShe et 2 Filed March 16, 1933 Patented Dec. 18, 1934 UNITED STATES PATENT OFFICE ELECTRICAL HEATING APPLIANCE OF THE OPEN-GRID OR BOILING-PLATE TYPE Neville Wallace Gilbert, Solihull, England 14 Claims.

This invention has reference to electric heating appliances of the boiling-plate type in which an open or exposed resistance element is mounted upon a refractory carrier and is connected in the secondary circuit of a step-down transformer to provide for the supply of high-amperage low- Voltage current to the said element.

In heaters of this'type, it has been proposed to use a single element in the form of a spiral or sinuous grid fashioned from heavy-gauge wire or strip metal (such as ni-chrome), but even where the mains current is transformed down to the lowest voltage consistent with heating efficiency, the potential diiference across the terminals of the grid is so high that short-circuiting of a portion of the grid-wire (such as by the bottom of a utensil making contact with two separated points thereof) may result in the said utensil being burnt or damaged by the short-circuited current. This risk is not eliminated by the insulating film or coating of oxide which. usually forms on ni-chrome resistances, whilst if, as a precaution against short-circuiting contact, the grid is arranged within deep channels in its refractory carrier, heating efiiciency is sacrificed by reason of the masking of a large area of the heat-radiating surface by the said carrier.

In the accompanying drawings,

Figure 1 is a plan of a boiling plate embodying improvements according to the present invention,

Figure 2 is a vertical section on the line 2-2 of Figure 1,

Figure 3 is a wiring diagram, and

Figure 4 is a plan of another form of heater.

According to my invention, and with the object of obviating the disadvantages above referred to, I propose to make-up a boiling-plate or like grid of two electrically-independent resistance elements and to arrange these elements respectively in secondary circuits of a double-secondary transformer that is adapted to supply to each of the said elements, a current of efiective heating value, but of such low voltage that the making of a short-circuit across either of the elements by a utensil bottom cannot result in damage to the said utensil. Further, and to obviate risk of shorting between the one element and the other, the said elements may be arranged in secondary circuits of opposite polarity.

A diagram explanatory of my system is shown in Figure 3 of the accompanying drawings, where a and I) represent the electrically-independent elements of a twin-resistor grid and a b are the secondary circuits of a double-secondary stepdown transformer which is so arranged that the currents in the secondary windings, and in the resistors, are of opposite or reverse polarities. The circuits a b and the grids a b, have a common connection at 0. Thus, as compared with the boiling-plate practice wherein a single-grid element is supplied with transformed-down current at a pressure in the order of 10 volts, the system as exemplified by the diagram enables the supply to each of two substantially-smaller elements of current at a pressure in the order of 5 volts, so that whilst the aggregate heating efiiciency of the said two elements can be made to equal or approximately equal that of a single grid containing an equivalent mass of wire or strip, an effective safeguard against dangerous short-circuiting is afforded by the provision which ensures that the potential difference between any two points in the grid system cannot exceed that which obtains between the terminals of either of the elements of the said system.

The secondary transformer windings with which the grid elements are respectively connected have the same number of turns around the transformer core sothat both the said elements carry current of the same voltage. Preferably, and as shown in the diagram the transformer comprises a single core and the primary circuit comprises four windings arranged in two pairs with a secondary winding interposed between each pair.

In this boiling-plate, an aperture in the top of the body is fitted with a refractory disc d which carries the resistance elements a b and is adapted also to serve as a heat reflector. The elements a b consist of spirals of heavy-gauge wire arranged as shown, and one end of each of them is connected to a common terminal 0 whilst their other ends are connected respectively with independent terminals or b the terminals being arranged in the secondary transformer circuits (t b as shown in the diagram, Figure 3.

Instead of using spiral elements as shown in Figures 1 and 2, the grid may be made up of two sinuously-formed elements a 17 arranged and con-- nected with the secondary terminals a c b as shown in Figure 4.

With a double-element grid arranged and supplied as above described, the elements may, with safety, be so mounted as to enable cooking and heating utensils to be placed directly thereon, but it is preferable to mount the said elements in the channels (1 of a refractory carrier as shown in Figures 1 and 2 in order that the said carrier may be utilized to maximum advantage as a reflector and to prevent loss of heat radiated downwardly from the grid. To ensure this, the elements are arranged, below the surface of the carrier d, in channels (1 which are substantially wider and deeper than the section of the wire so that air and heat may freely circulate around the whole of the grid. It is also preferred to sup port and locate the elements within their channels by a series of small sleeves e, made of nichrome or other heat-resistant metal and seated in recesses formed at intervals in the channels [3 These supports are notched at their upper ends to accommodate the wire, and being made of thin metal, their area of contact with the grid is negligible and the heat losses by conduction from the elements, through the supports to the reflector, are reduced to a minimum.

The terminals of the elements may be coupled by any form of connections which will pass the prescribed h gh-amperage two-voltage current, but where the boiling plate is embodied in a domestic cooking stove or the like, I prefer to adapt the arrangement shown in Figures 1 and 2 where the boiling plate is installed above a chambered casing f f into which water may be delivered and the terminals a b are connected with ccnductonrods a b of substantial crosssection, which are insulated from the casing and taken through a water-chamber f to the outside of the said casing where they are suitably connected with the secondaries al b of the transformer. With this arrangement, when the chamber is charged with water, the conductors a b are submerged and the heat generated thereon when the grid is in operation is not uselessly dissipated, but is utilized to heat the water in the chamber. By the provision of a tap in connection with the said chamber, water thus heated may be drawn off into a kettle or other utensil and subsequently brought quickly to the boil when the utensil is placed on the boilingplate.

I-laving described my invention, what I claim and desire to secure by Letters Patent is:

1. A boiling plate or similar heating grid comprising, in combination, two electrically-independent resistance elements, a double-secondary transformer, and connections between each resistance element and one secondary circuit of said double-secondary transformer, whereby to supply currents of opposite polarities to the respective resistance elements.

2. A boiling plate or similar heating grid comprising in combination, two electrically-independent resistance elements; a double-secondary transformer, whose secondaries are adapted to supply to each of the resistance elements currents of aggregate effective heating value, but of low and equal voltage; and connections between each resistance element and one secondary circuit of said double-secondary transformer to supply currents of opposite polarities and of low and equal voltage to each of the respective resistance elements, whereby a short circuit across either resistance element by a utensil placed thereon will not result in damage to said utensil.

3. A boiling plate or similar heating grid comprising in combination, two electrically-independent resistance elements, a double-secondary transformer, a terminal common to both secondaries, a common terminal connecting. an .end of one resistance element to an end of the other,

a connection between said last mentioned terminal and the terminal common to both secondaries of the double-secondary transformer, and connections between the opposite ends of each resistance element and one secondary circuit of the double-secondary transformer, whereby to supply said resistance elements with currents of opposite polarities.

4. A boiling plate or similar heating grid com prising in combination, two electrically-independent resistance elements, a refractory reflector having channels formed in its upper face adapted to receive therein the resistance elements, a double-secondary transformer, and connections between each resistance element and one secondary circuit of said double-secondary transformer, whereby to supply currents of opposite polarities to the respective resistance elements.

5. A boiling plate or similar heating grid comprising in combination, two electrically-independent resistance elements, a refractory reflector having channels formed on its upper face adapted to receive therein the resistance elements, spaced members of heat resisting material positioned within the channels to support said resistance elements within said channels, a double-secondary transformer, and connections between each resistance element and one secondary circuit of said double-secondary transformer, whereby to supply currents of opposite polarities to the respective resistance elements.

6. A boiling plate or similar heating grid comprising in combination, two electrically-independent resistance elements; a refractory reflector having channels formed on its upper face adapted to receive therein the resistance elements, said channels being substantially wider and deeper than the section of said resistance elements; a double-secondary transformer; and connections between each resistance element and one secondary circuit of said double-secondary transformer, whereby to supply currents of opposite polarities to the respective resistance elements.

7. A boiling plate or similar heating grid comprising in combination, two spirally-arranged, electrically-independent resistance elements, a double-secondary transformer, and connections between each spirally-arranged resistance element and one secondary circuit of said doublesecondary transformer, whereby to supply currents of opposite polarities to the respective spirally arranged resistance elements.

8. A boiling plate or similar heating grid comprising in combination, two spirally-arranged, electricallyindependent resistance elements, a refractory reflector having spirally-arranged channels formed in its upper face adapted to receive therein the spirally-arranged resistance elements, a double-secondary transformer, and connections between each resistance element and one secondary circuit of said double-secondary transformer, whereby to supply currents of opposite polarities to the respective spirally-arranged resistance elements.

9. A boiling plate or similar heating grid comprising in combination, two sinuously-arranged, electrically-hidependent resistance elements, a double-secondary transformer, and connections between each sinuously-arranged resistance element and one secondary circuit of said doublesecondary transformer, whereby to supply cur rents of opposite polarities to the respective sinuousl'y-arranged resistance elements.

10; A boiling plate or similar heating grid comprising in combination, two sinuously-arranged electrically-independent resistance elements, a

refractory reflector having sinuously-arranged channels formed in its upper face adapted to receive therein the sinuously-arranged resistance elements, a double-secondary transformer and connections between each sinuously-arranged resistance element and one secondary circuit of said double-secondary transformer, whereby to supply currents of opposite polarities to the respective sinuously-arranged resistance elements.

11. A boiling plate or similar heating grid comprising in combination, two electrically-independent resistance elements; a step-down transformer whose core is wound with four primary windings arranged in pairs, with a secondary winding interposed between each pair; and con nections between each resistance element and one of the secondary windings on said transformer, whereby to supply currents of opposite polarities to the respective resistance elements.

12. A boiling plate or similar heating grid comprising in combination, two electrically-independent resistance elements; a step-down transformer whose core is wound with four primary windings arranged in pairs, with a secondary winding interposed between each pair, each secondary winding having an equal number of turns; and connections between each resistance element and one of the secondary windings of said transformer, whereby to supply currents of equal voltage but of opposite polarities to the respective resistance elements.

13.A boiling plate or similar heating grid comprising a casing having a liquid-containing chamber therein, an electric heating member positioned above said casing, a set of terminals secured to a wall of said casing for connection to the heating element, a second set of terminals secured to a wall of said casing for connection to a source of current, conductors connecting the two sets of terminals and passing through the chamber and the liquid contained therein, said conductors heating said liquid when current is supplied to said heating member from said source, and means for withdrawing the liquid from the chamber.

14. A boiling plate or similar heating grid comprising a casing having a liquid-containing chamber therein; an electric heating member positioned above said casing and comprising two electrically-independent resistance elements; a double-secondary transformer; a set of three terminals secured to a wall of said casing, one of said terminals connecting an end of one resistance element to an end of the other, and two of said terminals secured each to one end of each resistance element; a second set of three terminals secured to a wall of said casing, one of said terminals connected to a terminal common to both secondaries of the double-secondary transformer, and each of the other two terminals connected to one secondary circuit of the doublesecondary transformer; conductors connecting the two sets of terminals and passing through the chamber and the liquid contained therein, whereby to supply currents of opposite polarities to the electrically-independent resistance ele-- ments, said conductors heating said liquid when current is supplied to said resistance elements from said double-secondary transformer; and means for withdrawing the liquid from the chamber. 

